The present invention relates to a method and apparatus for pilferage control. More particularly, it is directed to a method and apparatus for detecting the presence of a telltale element in an unauthorized zone.
In U.S. Pat. No. 3,895,368 issued July 15, 1975, for "Surveillance System and Method Utilizing Both Electrostatic and Electromagnetic Fields" and assigned to the same assignee as the present application, there is described a system with respect to which the present invention represents a significant improvement. Said patent describes a system wherein a microwave signal generator projects an electromagnetic wave into a space under surveillance to establish a first field. A pulse or frequency modulated low frequency generator is used to apply a voltage to a discontinuous conductor for establishing a second field, electrostatic in nature, throughout the space. Presence in the space of a miniature passive electromagnetic wave receptor-reradiator in the form of a semiconductive diode connected to a dipole antenna causes the reradiation of a low frequency component modulated on a microwave component as a carrier. The front end of a receiver system is tuned to the microwave frequency and feeds a suitable detector circuit responsive to the low frequency signal. A coincidence circuit energizes an alarm circuit whenever the detected signal coincides with the original modulation envelope being applied to the low frequency generator.
While said patented system represented a marked advance over the art then extant, it has been found to have certain limitations. In a typical installation, the electromagnetic and electrostatic fields are radiated from pedestals located on opposite sides of a doorway or exitway from an area to be supervised. The most common usage is to prevent pilferage from retail stores. In such case it is important that the surveillance zone be restricted to a small region proximate to the exit and prevented from overreaching or overranging into areas wherein it is desired to display merchandise or where normal traffic with unsold merchandise might take place. The usual radiation pattern from each pedestal constructed in accordance with the patented system is approximately circular centered around said pedestal. In order to cover the space between the pedestals, each pedestal must have a range at least greater than half the distance therebetween. However, while the pedestal has a range in front toward the opposite pedestal, it also has a range behind. It is the rear and lateral output or overranging that is undesirable and that is avoided by the present invention.